Questions are usually migrated when they are really good fits for the destination site. The questions you used as your examples are unfortunately mediocre at best.
(Disclaimer: for the two paragraphs below I speak as a Programmers moderator.)
Is it hard for older programmers to get a new job? was somewhat covered on Programmers here and here. I left a comment to that effect on the question. Since it just would be closed as a duplicate on Programmers, there is no reason to migrate it.
I want to start a web design site, Is it a realistic idea? could potentially have been migrated, but the question is pretty broad and unfocused. "Should I become a consultant at 17" is not a constructive question. "How do I start advertising my services" is better and perhaps could live on Answers.OnStartups.Com, but the question would need some editing first, I think.
Migrations done by community votes can only happen to 5 pre-defined sites and one of them is meta. Out of the remaining four, it's not always the case that there is
a place to migrate the question to.
In general, it's hard to expect for most Stack Overflow users to be familiar with other Stack Exchange sites. The moderators are more aware of what's out there and you will sometimes see questions moved by a moderator to various sites. They can also migrate to any site without having to rely on a pre-defined migration path.
It's important to try and find the right place for the question to get the best answers, but the bottom line is that moving questions around is disruptive to the asker and potentially the communities that will receive those questions. Migrations should be done with care and it may be better to close a question and (if possible) point the asker to another site and let them figure out for themselves if posting there would be more appropriate.